maybe the problem is a decline of "perceived" quality. The benz doors don't thunk like they used to. We want that solid sound every time we close the door to reassure us that it is a solid and safe place to be.
But not readily apparent to us are the improvements in the general structure of the car, in terms of torsional rigidity and flexure, and crash protection. For sure, also in terms of strength-to-weight ratio - either improved strength with the same weight, same strength with less weight, or improvements in both.
Longevity is also helped by rust protection/resistance, but this is not readily apparent from the appearance/feel of the materials - only time will tell.
i had the left front fender of my 1989 190e replaced because of rust perforation, and the unit they installed seemed like it had much less undercoating than the original fender. I hope that it is because they used better materials.
probably the parts of the car that contribute most to perceived quality are the interior materials that directly interface with the driver and passengers. Feel, texture and appearance count a lot, but only time will tell whether these will last. It was partly the move to more recyclable materials that increased the amount of interior plastic. I just hope they uphold the tradition of using long-wearing materials. My 12yr-old MB-Tex interior hardly has any wear.
Fit and finish however are another matter, as are the presence of squeaks and rattles, and slips in these areas are inexcusable.
having said these, i still want my solid-thunking doors ...